The present invention relates to a torque limiting device for an electric motor.
Electric motors are used in the most diverse and different areas. For example, the electric motors are used in electric tools and serve for driving an insertion tool. Depending on the area of application of the tool and/or the properties of the material to be treated and/or workpiece, it is frequently required that a predetermined torque must not be exceeded on the insertion tool, since otherwise the damages and/or destruction of the insertion tool or the workpiece can occur. For example, it is known in electric screwdrivers, depending on the density of the material which receives the screws and/or the size or geometry of the screws, to provide a torque increase in the course of the screwing process. In this case a torque can be reached such that the material structure of the screw and/or of the workpiece is loaded so that this material structure breaks up.
Situations are known, in which for example a screw is screwed with an electric screwdriver with a high rotary speed into a workpiece. When the end position of the screwing is reached from a high rotary speed, so that the screw head mechanically limits the further rotation of the screw, the screw, insertion tool and/workpiece can be damaged, or the screwing can be performed with an unacceptably high torque and thereby in a faulty way. In unfavorable cases, the material structure of the screw and/or insertion tool and/or workpiece can be overloaded with the suddenly occurring torque peaks so great, that the material structure breaks up. This situation is known as a so-called “hard screw case”.
Many measures have been taken to prevent the damage or breakup of the material in case of exceeding a maximum permissible torque. Mechanical safety clutches for this purpose are known, in which at reaching a preadjusted maximum permissible torque, the shaft associated with the electric motor is turned relative to the tool holder of the electric tool due to a predetermined slippage, so that the side associated with the tool can no longer be driven also when the motor or the parts associated with the motor, for example the transmission, run further. Mechanical safety couplings of this type have the disadvantage that they allow only a very inaccurate and in some cases only stepped adjustment of the maximum permissible torque. They further have a disadvantage that they are susceptible to a mechanical wear, and thereby the force transmission decreases in general and with increasing aging of the safety coupling. The through slippage with the adjusted maximum permissible torque is performed relatively inaccurately and within relatively great error intervals, so that the safety coupling interrupts the drive of the insertion tool at a torque which is lower than the preadjusted torque, and in many cases at reaching a higher torque, whereby a material damage can occur due to a torque-caused overloading.
Electronic torque limiting devices are also used, in which a motor current received from the electric motor is detected as a measure for the applied torque. Such devices operate inaccurately, since in particular at high spindle rotary speeds the torque which is actually occurs at the tool or at the workpiece with reaching of the predetermined motor current due to the predetermined maximum torque, is significantly greater than the adjusted limiting value. This phenomena occurs since after turning off of the motor current, due to kinetic energy in the rotating masses, these rotating masses because of their inertia have inclination to post-running. This post running is suddenly braked by the mechanical limit of the screw, which leads to formation of a torque which is independent from the drive. This phenomena acts in particular in the case of the above mentioned “hard screw case”, when the braking from a high rotary speed is performed suddenly and jerky.